Such a heat exchanger, has been disclosed in DE-OS No. 3803 947 and its U.S. equivalent Ser. No. 303,921 and DE-OS No. 3803948 and its U.S. equivalent Ser. No. 303,942 in which the duct segments are secured together by tension means arranged inside the ducts, said tension means being formed as tension rods or as tension tubes. In this way, both the manufacture of the heat exchanger and its maintenance are considerably simplified, since only individual duct segments must be processed or replaced. Also, there are no sealing problems created when the hot gases flow externally of the manifold ducts, since the ducts heat up first to a greater extent than the internally arranged tension means, and thus tightness at the joints of adjacent duct segments is guaranteed during the start-up phase of operation.
A disadvantaqe of this arranqement is that the tension means in the ducts causes a considerable flow resistance to the fluid flowing in the ducts, and this resistance may lead to a considerable reduction in the total efficiency of the plant driven by the heat exchanger. A further disadvantage is that during certain operating conditions, for example, a reduction of full load to partial load, due to an externally reduced flow temperature, the manifold ducts cool and contract whereas the tension means inside the ducts remain at the previous high temperature whereby the tensioning is of the duct segments is relaxed and leakage may occur under unfavorable circumstances between adjacent duct segments.
A further disadvantage is that the above described construction is suitable only for heat exchangers of smaller construction size, since with larger heat exchangers the load on the manifold ducts due to the intrinsic weight of the matrix of heat exchange tubes and the dynamic loads that occur would be so high that several supports would be necessary in the radial direction along the length of each manifold duct. With rigid joining flanges for the duct segments, a closed tube connection is produced of considerable structural length. Because of temperature gradients over the cross section of the ducts, a bending of the ducts along the longitudinal axis occurs. Due to this bending and due to its multiple supports, the support forces are indeterminate. This may lead to prevention of longitudinal expansion and considerable loads will be developed over the cross section of the ducts.
Considerable forces are produced in a closed tube construction by dynamic loads acting in the axial direction of the manifold ducts and these forces place a considerable load on the ducts. A further disadvantage is that manifold ducts, which have at least one closed end, and whose axial support is arranged at a substantial distance from this closed end will have considerable load developed due to axial forces acting on the closed end, of the duct as a consequence of internal pressure. This has a substantial effect on the strength of the support tubes which have a perforated cross section.